5^4 



NA TURE 



[October 8, 1903 



IV. Wealden Floras, — Geographical Distribution of Characteristic Types. 



Characteristic Types 



Equisetales 

 Equisetites ... 



FiLICALES 

 • Ouychiopsis 

 Matonidium 

 Cladophlebis 

 Sphenopteris 

 Weichselia ... 

 Taeniopteris 

 Laccopleris . . . 

 Gleichenites 



GlNKGOAI.ES 

 Baiera \ 

 Ginkgo J 



^ONIFERALES 

 Sphenoleptdiu m 

 Araucarites 

 Pinites 



Oycadophyta 



Nilssonia ... 

 Otozatnites ... 

 Zamites 

 Bennetfiites . . . 



'n. Temperate N. Sub-tropical 



« 2 3 4 5 ^ i 7 



10 II 12 13 



Tropical 



S. Sub- ~ „, 

 tropical i^ lemperate 



9 I 20 2X 22 



Agathis, the former including ten species occurring in 

 South America and Australia, and the latter comprising 

 four species which flourish in the Malay Archipelago, New 

 Zealand, the Philippines, North-East Australia, and else- 

 where. Sir William Thiselton-Dyer pointed out, in a 

 lecture on plant-distribution, delivered in 1878, that the 

 genus Araucaria appears to have been extinct in a wild 

 statr north of the Equator since the Jurassic epoch. 

 Additional confirmation of the important status of this 

 section of the Coniferae is afforded by the abundance of 

 petrified wood exhibiting Araucarian features, in both 

 Jurassic and Wealden rocks. There is good reason to 

 believe that the well-known Whitby jet was formed by' the 

 alteration of blocks of Araucarian wood drifted from forest- 

 clad slopes overlooking a Jurassic estuary that occupied the 

 site of the moors and headlands of North-East Yorkshire. 

 Among familiar Jurassic genera, mention must be made of 

 the genus Brachyphyllum, including species referred by 

 some authors to Athrotaxites, represented by fragments of 

 leafy twigs and branches bearing a striking resemblance 

 to those of the isolated Tasmanian genus Athrotaxis. 

 Omitting further reference to the various indications 

 afforded by a study of Mesozoic Conifers as to the former 

 extension of many of the more isolated recent types, we 

 may present in a tabular form an epitome of the past and 

 present range of the Araucariese : — 



b. Cycads. 

 One of the most striking features of the Mesozoic vegeta- 

 tion is the abundance and wide distribution of Cycadean 

 plants. To-day the Cycads or Sago-Palms are represented 

 by ten genera and about eighty species ; they are plants 

 which occupy a subordinate position in modern floras, and 

 occur for the most part as solitary types in tropical lati- 

 tudes, never growing together in sufficiently large numbers 

 to constitute a dominant feature in the vegetation. Cycads 

 have long attracted attention as exhibiting morphological 

 features of considerable interest. During the last few years 

 the work of Ikeno, Webber, and Lang has shown us that 

 the pollen of Cycas, Zamia, Stangeria, and probably of the 

 other recent genera, produce spirally ciliated motile sperm- 

 atozoids, the type of male cell previously regarded as con- 

 stituting one of the well-defined distinctions between the 

 Vascular Cryptogams and the Seed-bearing plants. The 

 study of Palaeozoic plants has done even more to break 

 down the artificial barrier between Cycads and Vascular 

 Cryptogams, by demonstrating beyond all reasonable doubt 

 that our modern Cycads represent a small group of survivals 

 descended from ancestors common to themselves and the 

 ferns. Cycadean plants must have been among the 

 commonest meiribers of Mesozoic floras. Before the end 

 of the Palaeozoic era there existed plants bearing pinnate 

 fronds similar to those of recent species of Cycadacese, and 



'' Geographical Distribution of Past and Present Araucarie^. 



NO. 1 77 1, VOL. 68] 



